I. Field of the Invention
The present invention relates to signal amplifiers. More specifically, the present invention relates to methods and circuit arrangements for providing highly efficient, linear signal amplification over a wide dynamic range by employing multiple parallel amplifying devices.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation (AM) modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307 entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, the disclosure thereof incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations (also known as cell-site stations, or for short cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques. In a CDMA system, increases in system capacity may be realized by controlling the transmitter power of the portable units associated with each user so as to reduce interference to other system users.
In a terrestrial CDMA cellular communication system it is extremely desirable to maximize the capacity in terms of the number of simultaneous communication links capable of being supported by a given system bandwidth. System capacity can be maximized if the transmitter power of each portable unit is controlled such that the transmitted signal arrives at the cell-site receiver with the minimal signal to noise interference ratio which allows acceptable data recovery. If a signal transmitted by a portable unit arrives at the cell-site receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications. If, on the other hand, acceptable communication is established by setting the mobile unit transmitted signal at a power level that is too high when received at the cell site receiver, interference will occur with other mobile unit transmitted signals that are sharing the same channel, i.e. bandwidth. This interference may adversely affect communications with other portable units unless the total number of communicating portable units is reduced.
The signals received from each portable unit at the cell-site station are measured, and the measurement results compared with a desired power level. Based on this comparison the cell-site determines the deviation in the received power level from that which is necessary to maintain the desired communications. Preferably the desired power level is a minimum power level necessary to maintain quality communications so as to result in a reduction in system interference.
The cell-site station then transmits a power control command signal to each system user so as to adjust or "fine tune" the transmit power of the portable unit. This command signal is used by the portable unit to change the transmit power level closer to a level required to sustain communication on the reverse link between the portable unit and the cell-site. As channel conditions change, typically due to motion of the portable unit, both the portable unit receiver power measurement and the power control feedback from the cell-site station continually readjust the transmit power level so as to maintain a proper power level.
The utilization of these types of power control techniques requires that the portable unit transmitter be capable of linear operation over a relatively wide dynamic range. Since existing portable units operate on battery power, it is also necessary that the transmitter power amplifier be capable of efficient, linear operation over the dynamic range typical of CDMA communication systems. Since conventional power amplifier designs, both variable gain and fixed gain, have been found to lack the requisite efficiency and linearity over a wide dynamic range, there exists a need for a power amplifier capable of providing this type of performance.